1. Field of the Invention
The present invention relates to a thermosetting resin composition. More specifically, the present invention relates to a thermosetting resin composition obtained by blending into a curable thermosetting resin a cured organosiloxane composition in powdered form. The excellent adhesion between these two ingredients results in a cured composition with reduced water infiltrability compared with compositions containing prior art cured organosiloxane powders, an excellent flexibility, a low coefficient of thermal expansion and a small mold shrinkage.
2. Description of the Prior Art
Thermosetting resin compositions are widely used as insulating materials for various electric and electronic components and are fabricated by means such as transfer molding, injection molding, potting, casting, powder coating, immersion coating and dripping. These compositions exhibit excellent electrical properties, including dielectric properties, volume resistivity, and insulation breakdown strength, and excellent mechanical properties, including bending strength, compressive strength, and impact strength.
While the coefficients of thermal expansion and shrinkage ratios of the elements of electric and electronic parts are small, the resins have large values. The large difference between these values and the rigidity of the resins results in excessive internal stress being applied to the elements or other structural materials of the electric or electronic part during molding or baking or any subsequent thermal cycle. As a consequence, when the resins are used to seal, for example, an electric or electronic component, the element frequently will not function properly or part of the element may be broken. Furthermore, this difference in coefficients of thermal expansion and shrinkage ratios cause cracking in the thermosetting resin itself, and may cause the generation of gaps between the electric or electronic part and the thermosetting resin. Water can infiltrate into such gaps and can result in degradation of the element.
Prior efforts at improving the properties of thermosetting resins have not had as their goal reducing the aforementioned differences in the coefficient of thermal expansion or post-molding shrinkage ratio. For example, Japanese Patent Publication 52-36534 [36,534/77] is concerned with improving the surface lubricity of resin moldings by blending organopolysilsesquioxane powder into phenolic resins.
Japanese Laid Open Patent Application [Kokai] Number 52-14643 [14,643/77] is concerned with improving the abrasion resistance of synthetic resins against metal by use of a very finely divided cured material composed principally of an organopolysiloxane and an inorganic filler as the filler for the synthetic resin. However, the cured compositions described in both of the aforementioned patent publications exhibit unsatisfactory thermal expansion, post-molding shrinkage ratio and flexural modulus of elasticity.
Japanese Laid Open Patent Application Number 58-219218 [219,218/83], describes the present inventors efforts to solve these problems by blending a finely divided cured material containing at least 10 percent by weight of a linear siloxane moiety into the thermosetting resin. Furthermore, in Japanese Laid Open Patent Application Number 59-96122 [96,122/84], the present inventors propose using spherical particles of a material that has been cured into an elastomer during spraying. However, when the aforementioned finely divided cured material or microfine spherical cured material is blended into a thermosetting resin, water easily infiltrates into the spaces between the very finely particulate cured material and the thermosetting resin due to poor bonding to the thermosetting resin. This is particularly true for very finely particulate cured material consisting of 100 wt % linear siloxane moiety.
U.S. Pat. No. 4,663,397, which issued on May 5, 1987 in the names of Y. Morita, one of the present inventors, and A. Shirahata describes thermosetting resin compositions comprising a curable organic thermosetting resin and a cured polyorganosiloxane resin having repeating units of the formulae R.sup.1 R.sup.2 R.sup.3 SiO.sub.0.5 and R.sup.4 SiO.sub.1.5. Any additional repeating units are of the formula R.sup.5 R.sup.6 SiO, SiO.sub.2 and/or are derived from block copolymers of polyorganosiloxane resins and organic resins. The substituents represented by R.sup.1 -R.sup.6 are individually selected from hydrogen atoms and monovalent hydrocarbon radicals. These compositions suffer from the same disadvantages as other prior art compositions with respect to poor adhesion between the two types of materials present in the molded composition. The poor adhesion allows water to infiltrate into the final cured composition.